In vehicles where the engine is controlled by an engine electronic control unit (referred to hereinafter as ECU) comprising a microprocessor, when an ignition switch is turned to a Start position by an ignition key, a starter cranks a crankshaft, a pulse signal is sent from the ECU to a fuel injection valve, fuel is injected from the valve into the engine, and the engine starts.
However if the ignition switch is shorted, the engine can still be started and the vehicle can be operated even without the ignition key. This mechanism was therefore inadequate from the viewpoint of preventing vehicle theft.
Tokkai Sho 64-56253 published by the Japanese Patent Office in 1989 discloses a vehicle antitheft device wherein a electronic code is built into the key. After the engine is started by the key, the electronic code sent from the key is compared with a code stored in the ECU, and if the codes do not match, engine operation is prevented.
If there is another electronic device capable of communicating with the ECU and that device sends a code to the ECU, there is very little probability that the codes will match. However, if a program built into the device repeatedly sends codes to the ECU, there is a risk that the codes will eventually match.